Development of electrochemical method to detect bacterial count, Listeria monocytogenes, and somatic cell count in raw milk

• To develop an easy, low-cost and rapid quantitative electrochemical detection method for measuring the bacterial count, Listeria monocytogenes detection and somatic cell count (SCC) in raw milk.
• Compared to plate counting method, the electrochemical method is simpler to operate, faster and can reduce human errors for determination of the bacterial count in raw milk.
• Only Listeria monocytogenes could grow selectively in Fraser broth owing to the selective nutrients addition of ferric ammonium citrate and esculin, the current signal resulting from the microbial growth was monitored more apparently.
• SCC was determined by measuring current signal produced by the expression of lactate dehydrogenase (LDH) activity, a biomarker also for diagnosing mastitis.

Bacterial count, Listeria monocytogenes detection and somatic cell count (SCC) are generally recognized as indexes of raw milk quality which could affect the price and the shelf-life of dairy products. This research was aimed at developing an easy, low-cost and rapid quantitative electrochemical detection method for measuring the three indexes simultaneously. Through the electrochemical sensor, bacterial count or L. monocytogenes detection was determined by evaluating the electric-current originated from microbial activity. SCC was determined by measuring current signal produced by the expression of lactate dehydrogenase (LDH) activity, a biomarker also for diagnosing mastitis. The research indicated that the appropriate conditions for applying the biosensor to bacterial count detection ranging from 102 to 108 CFU/ml within detection time (DT) of 1–10 h are 35 °C at 1 V and that the optimal conditions for L. monocytogenes detection are 37 °C at 1 V, under pH 9.4, associated with a Fraser selective broth. The optimal conditions for SCC detection ranging from 350 to 780 thousand SCC/ml (R2 = 0.907) within 60 s (DT) are 42 °C at 0.5 V, under pH 9.0. The biosensor is shown to be a promising approach for the detections in raw milk.

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